Massive Spirals Research Articles

Accelerated Structure Formation: The Early Emergence of Massive Galaxies and Clusters of Galaxies

Galaxies in the early Universe appear to have grown too big too fast, assembling into massive, monolithic objects more rapidly than anticipated in the hierarchical Lambda cold dark matter (ΛCDM) structure formation paradigm. The available photometric data are consistent with there being a population of massive galaxies that form early (z ≳ 10) and quench rapidly over a short (≲1 Gyr) timescale, consistent with the traditional picture for the evolution of giant elliptical galaxies. Similarly, kinematic observations as a function of redshift show that massive spirals and their scaling relations were in place at early times. Explaining the early emergence of massive galaxies requires either an extremely efficient conversion of baryons into stars at z > 10 or a more rapid assembly of baryons than anticipated in ΛCDM. The latter possibility was explicitly predicted in advance by modified Newtonian dynamics (MOND). We discuss some further predictions of MOND, such as the early emergence of clusters of galaxies and early reionization.

Quantifying the stellar ages of dynamically separated bulges and disks of CALIFA spiral galaxies

We employ a recently developed population-orbit superposition technique to simultaneously fit the stellar kinematic and age maps of 82 CALIFA spiral galaxies and obtain the ages of stars in different dynamical structures. We first evaluated the capabilities of this method on CALIFA-like mock data created from the Auriga simulations. The recovered mean ages of dynamically cold, warm, and hot components match the true values well, with an observational error of up to 20% in the mock age maps. For CALIFA spiral galaxies, we find that the stellar ages of the cold, warm, and hot components all increase with the stellar mass of the galaxies, from tcold ~ 2.2 Gyr, twarm ~ 2.3 Gyr, and thot ~ 2.6 Gyr for galaxies with stellar mass M* < 1010 M⊙, to tcold ~ 4.0 Gyr, twarm ~ 5.1 Gyr, and thot ~ 5.9 Gyr for galaxies with M* > 1011 M⊙. About 80% of the galaxies in our sample have thot > tcold, and the mean values of thot − tcold also increase with stellar mass, from 0.7−0.2+0.6 Gyr in low-mass galaxies (108.9 M⊙ < M* ≤ 1010.5 M⊙) to 1.7−0.2+0.7 Gyr in high-mass galaxies (1010.5 M⊙ < M* < 1011.3 M⊙). The stellar age is younger in disks than in bulges, on average. This suggests that either the disks formed later and/or that they experienced a more prolonged and extensive period of star formation. Lower-mass spiral galaxies have younger bulges and younger disks, while higher-mass spiral galaxies generally have older bulges, and their disks span a wide range of ages. This is consistent with the scenario in which the bulges in more massive spirals formed earlier than those in less massive spirals.

Probing star formation in five of the most massive spiral galaxies observed through ASTROSAT UltraViolet Imaging Telescope

ABSTRACT We present highly resolved and sensitive imaging of the five nearby massive spiral galaxies (with rotation velocities $\rm \gt 300\, km\, s^{-1}$) observed by the UltraViolet Imaging Telescope onboard India’s multiwavelength astronomy satellite ASTROSAT, along with other archival observations. These massive spirals show a far-ultraviolet star formation rate in the range of ∼ 1.4 – 13.7 ${\rm M}_{\odot } \, \mathrm{yr}^{-1}$ and fall in the ‘Green Valley’ region with a specific star formation rate within ∼ 10−11.5 – 10−10.5 yr−1. Moreover, the mean star formation rate density of the highly resolved star-forming clumps of these objects is in the range 0.011 – 0.098 ${\rm M}_{\odot }\, \mathrm{ yr}^{-1}\, \mathrm{kpc}^{-2}$, signifying localized star formation. From the spectral energy distributions, under the assumption of a delayed star formation model, we show that the star formation of these objects had peaked in the period of ∼ 0.8 – 2.8 Gyr after the ‘Big Bang’ and the object that has experienced the peak sooner after the ‘Big Bang’ show relatively less star-forming activity at z ∼ 0 and falls below the main-sequence relation for a stellar content of $\rm \gtrsim 10^{11} \, {\rm M}_{\odot }$. We also show that these objects accumulated much of their stellar mass in the early period of evolution with ∼ 31 – 42 per cent of the total stellar mass obtained in a time of (1/16) – (1/5)th the age of the Universe. We estimate that these massive objects convert their halo baryons into stars with efficiencies falling between ∼ 7 and 31 per cent.

Cold gas disks in main-sequence galaxies at cosmic noon: Low turbulence, flat rotation curves, and disk-halo degeneracy

We study the dynamics of cold molecular gas in two main-sequence galaxies at cosmic noon (zC-488879 at z ≃ 1.47 and zC-400569 at z ≃ 2.24) using new high-resolution ALMA observations of multiple 12CO transitions. For zC-400569 we also reanalyze high-quality Hα data from the SINS/zC-SINF survey. We find that (1) both galaxies have regularly rotating CO disks and their rotation curves are flat out to ∼8 kpc contrary to previous results pointing to outer declines in the rotation speed Vrot; (2) the intrinsic velocity dispersions are low (σCO ≲ 15 km s−1 for CO and σHα ≲ 37 km s−1 for Hα) and imply Vrot/σCO ≳ 17 − 22 yielding no significant pressure support; (3) mass models using HST images display a severe disk-halo degeneracy, that is models with inner baryon dominance and models with “cuspy” dark matter halos can fit the rotation curves equally well due to the uncertainties on stellar and gas masses; and (4) Milgromian dynamics (MOND) can successfully fit the rotation curves with the same acceleration scale a0 measured at z ≃ 0. The question of the amount and distribution of dark matter in high-z galaxies remains unsettled due to the limited spatial extent of the available kinematic data; we discuss the suitability of various emission lines to trace extended rotation curves at high z. Nevertheless, the properties of these two high-z galaxies (high Vrot/σV ratios, inner rotation curve shapes, bulge-to-total mass ratios) are remarkably similar to those of massive spirals at z ≃ 0, suggesting weak dynamical evolution over more than 10 Gyr of the Universe’s lifetime.

H i content of massive red spiral galaxies observed by FAST

ABSTRACT A sample of 279 massive red spirals was selected optically by Guo et al., among which 166 galaxies have been observed by the ALFALFA survey. In this work, we observe H i content of the rest 113 massive red spiral galaxies using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). 75 of the 113 galaxies have H i detection with a signal-to-noise ratio (S/N) greater than 4.7. Compared with the red spirals in the same sample that have been observed by the ALFALFA survey, galaxies observed by FAST have on average a higher S/N, and reach to a lower H i mass. To investigate why many red spirals contain a significant amount of H i mass, we check colour profiles of the massive red spirals using images observed by the DESI Legacy Imaging Surveys. We find that galaxies with H i detection have bluer outer discs than the galaxies without H i detection, for both ALFALFA and FAST samples. For galaxies with H i detection, there exists a clear correlation between galaxy H i mass and g-r colour at outer radius: galaxies with higher H i masses have bluer outer discs. The results indicate that optically selected massive red spirals are not fully quenched, and the H i gas observed in many of the galaxies may exist in their outer blue discs.

MIGHTEE-H i: the H i size–mass relation over the last billion years

ABSTRACT We present the observed H i size–mass relation of 204 galaxies from the MIGHTEE Survey Early Science data. The high sensitivity of MeerKAT allows us to detect galaxies spanning more than 4 orders of magnitude in H i mass, ranging from dwarf galaxies to massive spirals, and including all morphological types. This is the first time the relation has been explored on a blind homogeneous data set that extends over a previously unexplored redshift range of 0 < z < 0.084, i.e. a period of around one billion years in cosmic time. The sample follows the same tight logarithmic relation derived from previous work, between the diameter ($D_{\rm H\, \small {\rm I}}$) and the mass ($M_{\rm H\, \small {\rm I}}$) of H i discs. We measure a slope of 0.501 ± 0.008, an intercept of $-3.252^{+0.073}_{-0.074}$, and an observed scatter of 0.057 dex. For the first time, we quantify the intrinsic scatter of 0.054 ± 0.003 dex (${\sim } 10 {{\ \rm per\ cent}}$), which provides a constraint for cosmological simulations of galaxy formation and evolution. We derive the relation as a function of galaxy type and find that their intrinsic scatters and slopes are consistent within the errors. We also calculate the $D_{\rm H\, \small {\rm I}}{ \!-\! }M_{\rm H\, \small {\rm I}}$ relation for two redshift bins and do not find any evidence for evolution with redshift. These results suggest that over a period of one billion years in look-back time, galaxy discs have not undergone significant evolution in their gas distribution and mean surface mass density, indicating a lack of dependence on both morphological type and redshift.

Disentangling the formation mechanisms of nuclear star clusters

Nuclear star clusters (NSCs) are massive star clusters found ubiquitously in the centres of galaxies, from the dwarf regime to massive ellipticals and spirals. The fraction of nucleated galaxies is as high as > 90% at Mgal ∼ 109 M⊙. However, how NSC formation mechanisms work in different regimes and what determines galaxy nucleation is still unclear. The dissipationless accretion of infalling globular clusters (GCs) and the in situ formation of stars directly at the galactic centre likely operate to grow NSCs in most galaxies; however, their efficiency has been difficult to assess observationally. Here, we provide, for the first time, a quantitative determination of the relative strength of these processes in the build-up of individual NSCs. Using a semi-analytical model of NSC formation based on the orbital evolution of inspiraling GCs, together with observed NSC and GC system properties, we derived the mass fraction of in situ born stars fin, NSC for 119 galaxies with masses from 3 × 107 to 3 × 1011 M⊙, in the Local Volume, the Fornax, and Virgo galaxy clusters. Our analysis reveals that the NSC mass, as well as the ratio of NSC to the total GC system mass, are strong indicators of the dominant NSC formation channel, and not the total galaxy stellar mass as previously suggested. More massive NSCs formed predominantly via the in situ formation of stars (fin, NSC ∼ 0.9), while the lower-mass NSCs are expected to have formed predominantly through the merger of GCs (fin, NSC ∼ 0.2). The results of this simple model are in agreement with recent independent estimates of the dominant NSC formation channel from recent stellar population analysis.

Quenching, bursting, and galaxy shapes: colour transformation as a function of morphology

ABSTRACT Different mechanisms for quenching star formation in galaxies are commonly invoked in the literature, but the relative impact of each one at different cosmic epochs is still unknown. In particular, the relation between these processes and morphological transformation remains poorly understood. In this work, we measure the effectiveness of changes in star formation rates by analysing a new parameter, the Star Formation Acceleration (SFA), as a function of galaxy morphology. This methodology is capable of identifying both bursting and quenching episodes that occurred in the preceding 300 Myr. We use morphological classification catalogues based on Deep Learning techniques. Our final sample has ∼14 200 spirals and ∼2500 ellipticals. We find that elliptical galaxies in the transition region have median shorter quenching time-scales (τ < 1 Gyr) than spirals (τ ≥ 1 Gyr). This result conforms to the scenario in which major mergers and other violent processes play a fundamental role in galaxy evolution for most ellipticals, not only quenching star formation more rapidly but also playing a role in morphological transformation. We also find that ∼two-thirds of galaxies bursting in the green valley in our sample are massive spirals ($M_\star \ge 10^{11.0}\, \mathrm{M}_\odot$) with signs of disturbance. This is in accordance with the scenario where low-mass galaxies are losing their gas in an interaction with a massive galaxy: while the former is quenching, the last is being refueled and going through a burst, showing signs of recent interaction.

Star Formation Histories of Massive Red Spiral Galaxies in the Local Universe

Abstract We investigate the star formation histories (SFHs) of massive red spiral galaxies with stellar mass M * > 1010.5 M ⊙, and make comparisons with blue spirals and red ellipticals of similar masses. We make use of the integral field spectroscopy from the Sloan Digital Sky Survey IV/DR15 MaNGA sample, and estimate spatially resolved SFHs and stellar population properties of each galaxy by applying a Bayesian spectral fitting code to the MaNGA spectra. We find that both red spirals and red ellipticals have experienced only one major star formation episode at early times, and the result is independent of the adopted SFH model. On average, more than half of their stellar masses were formed >10 Gyr ago, and more than 90% were formed >6 Gyr ago. The two types of galaxies show similarly flat profiles in a variety of stellar population parameters: old stellar ages indicated by D4000 (the spectral break at around 4000 Å), high stellar metallicities, large Mgb/Fe ratios indicating fast formation, and little stellar dust attenuation. In contrast, although blue spirals also formed their central regions >10 Gyr ago, both their central regions and outer disks continuously form stars over a long timescale. Our results imply that massive red spirals are likely to share some common processes of formation (and possibly quenching) with massive red ellipticals in the sense that both types were formed at z > 2 through a fast formation process. Possible mechanisms for the formation and quenching of massive red spirals are discussed.

Revisiting the tension between fast bars and the ΛCDM paradigm

The pattern speed with which galactic bars rotate is intimately linked to the amount of dark matter in the inner regions of their host galaxies. In particular, dark matter haloes act to slow down bars via torques exerted through dynamical friction. Observational studies of barred galaxies tend to find that bars rotate fast, while hydrodynamical cosmological simulations of galaxy formation and evolution in the Lambda cold dark matter (ΛCDM) framework have previously found that bars slow down excessively. This has led to a growing tension between fast bars and the ΛCDM cosmological paradigm. In this study we revisit this issue, using the Auriga suite of high-resolution, magneto-hydrodynamical cosmological zoom-in simulations of galaxy formation and evolution in the ΛCDM framework, finding that bars remain fast down to z = 0. In Auriga, bars form in galaxies that have higher stellar-to-dark matter ratios and are more baryon-dominated than in previous cosmological simulations; this suggests that in order for bars to remain fast, massive spiral galaxies must lie above the commonly used abundance matching relation. While this reduces the aforementioned tension between the rotation speed of bars and ΛCDM, it accentuates the recently reported discrepancy between the dynamically inferred stellar-to-dark matter ratios of massive spirals and those inferred from abundance matching. Our results highlight the potential of using bar dynamics to constrain models of galaxy formation and evolution.

The baryonic specific angular momentum of disc galaxies

Aims.Specific angular momentum (the angular momentum per unit mass,j = J/M) is one of the key parameters that control the evolution of galaxies, and it is closely related with the coupling between dark and visible matter. In this work, we aim to derive the baryonic (stars plus atomic gas) specific angular momentum of disc galaxies and study its relation with the dark matter specific angular momentum.Methods.Using a combination of high-quality H Irotation curves, H Isurface densities, and near-infrared surface brightness profiles, we homogeneously measure the stellar (j*) and gas (jgas) specific angular momenta for a large sample of nearby disc galaxies. This allows us to determine the baryonic specific angular momentum (jbar) with high accuracy and across a very wide range of masses.Results.We confirm that thej* − M*relation is an unbroken power-law from 7 ≲ log(M*/M⊙) ≲ 11.5, with a slope 0.54 ± 0.02, setting a stronger constraint at dwarf galaxy scales than previous determinations. Concerning the gas component, we find that thejgas − Mgasrelation is also an unbroken power-law from 6 ≲ log(Mgas/M⊙) ≲ 11, with a steeper slope of 1.02 ± 0.04. Regarding the baryonic relation, our data support a correlation characterized by a single power-law with a slope 0.60 ± 0.02. Our analysis shows that our most massive spirals and smallest dwarfs lie along the samejbar − Mbarsequence. While the relations are tight and unbroken, we find internal correlations inside them: At fixedM*, galaxies with largerj*have larger disc scale lengths, and at fixedMbar, gas-poor galaxies have lowerjbarthan expected. We estimate the retained fraction of baryonic specific angular momentum,fj, bar, finding it constant across our entire mass range with a value of ∼0.6, indicating that the baryonic specific angular momentum of present-day disc galaxies is comparable to the initial specific angular momentum of their dark matter haloes. In general, these results set important constraints for hydrodynamical simulations and semi-analytical models that aim to reproduce galaxies with realistic specific angular momenta.

How to disentangle geometry and mass-loss rate from AGB-star spectral energy distributions

Context. High-angular-resolution observations of asymptotic giant branch (AGB) stars often reveal non-spherical morphologies for the gas and dust envelopes. Aims. We aim to make a pilot study to quantify the impact of different geometries (spherically symmetric, spiral-shaped, and disc-shaped) of the dust component of AGB envelopes on spectral energy distributions (SEDs), mass estimates, and subsequent mass-loss rate (MLR) estimates. We also estimate the error made on the MLR if the SED is fitted by an inappropriate geometrical model. Methods. We use the three-dimensional Monte-Carlo-based radiative-transfer code RADMC-3D to simulate emission from dusty envelopes with different geometries (but fixed spatial extension). We compare these predictions with each other, and with the SED of the AGB star EP Aqr that we use as a benchmark since its envelope is disc-like and known to harbour spiral arms, as seen in CO. Results. The SEDs involving the most massive envelopes are those for which the different geometries have the largest impact, primarily on the silicate features at 10 and 18 μm. These different shapes originate from large differences in optical depths. Massive spirals and discs appear akin to black bodies. Optically thick edge-on spirals and discs (with dust masses of 10−4 and 10−5 M⊙) exhibit black-body SEDs that appear cooler than those from face-on structures and spheres of the same mass, while optically thick face-on distributions appear as warmer emission. We find that our more realistic models, combined spherical and spiral distributions, are 0.1 to 0.5 times less massive than spheres with similar SEDs. More extreme, less realistic scenarios give that spirals and discs are 0.01 to 0.05 times less massive than corresponding spheres. This means that adopting the wrong geometry for an AGB circumstellar envelope may result in a MLR that is incorrect by as much as one to two orders of magnitude when derived from SED fitting.

Lessons on Star-forming Ultra-diffuse Galaxies from the Stacked Spectra of the Sloan Digital Sky Survey

Abstract We investigate the on-average properties for 28 star-forming ultra-diffuse galaxies (UDGs) located in low-density environments, by stacking their spectra from the Sloan Digital Sky Survey. These relatively isolated UDGs, with stellar masses of , have the on-average total stellar metallicity [M/H] ∼ −0.82 ± 0.14, iron metallicity [Fe/H] ∼ −1.00 ± 0.16, stellar age t * ∼ 5.2 ± 0.5 Gyr, α-enhancement [α/Fe] ∼ 0.24 ± 0.10, and oxygen abundance 12+log(O/H) ∼ 8.16 ± 0.06, as well as central stellar velocity dispersion 54 ± 12 km s−1. On the star formation rate versus stellar mass diagram, these UDGs are located lower than the extrapolated star-forming main sequence from the massive spirals, but roughly follow the main sequence of low-surface-brightness dwarf galaxies. We find that these star-forming UDGs are not particularly metal-poor or metal-rich for their stellar masses, as compared with the metallicity–mass relations of the nearby typical dwarfs. With the UDG data of this work and previous studies, we also find a coarse correlation between [Fe/H] and magnesium-element enhancement [Mg/Fe] for UDGs: [Mg/Fe] ≃ −0.43(±0.26) [Fe/H] −0.14(±0.40).

Massive disc galaxies too dominated by dark matter in cosmological hydrodynamical simulations

We investigated the disc-halo connection in massive (M⋆ > 5 × 1010 M⊙) disc galaxies from the cosmological hydrodynamical simulations EAGLE and IllustrisTNG, and compared this connection with that inferred from the study of H I rotation curves in nearby massive spirals from the Spitzer Photometry and Accurate Rotation Curves dataset. We find that discrepancies between the simulated and observed discs arise both on global and local scales. Globally, the simulated discs inhabit halos that are a factor ∼4 (in EAGLE) and ∼2 (in IllustrisTNG) more massive than those derived from the rotation curve analysis of the observed dataset. We also used synthetic rotation curves of the simulated discs to demonstrate that the recovery of the halo masses from rotation curves are not systematically biased. We find that the simulations predict systems dominated by dark matter with stellar-to-total enclosed mass ratios that are a factor of 1.5−2 smaller than real galaxies at all radii. This is an alternative manifestation of the ‘failed feedback problem,’ since it indicates that simulated halos hosting massive discs have been too inefficient at converting their baryons into stars, possibly owing to an overly efficient stellar and/or AGN feedback implementation.

Toward an Understanding of the Massive Red Spiral Galaxy Formation

Abstract To understand the formation and quenching processes of local massive red spiral galaxies with M * > 1010.5 M ⊙, we perform a statistical analysis of their spectroscopic and structural properties and compare them with elliptical and blue spiral galaxies of similar mass. The sample was selected from the stellar mass catalog of galaxies in SDSS DR7, according to their locations on the u − r color−stellar mass diagram. We find that red spirals harbor compact cores with high stellar mass surface densities measured by Σ1 and are bulge dominated. Particularly, the red spirals, especially their bulges, follow the Σ1–M * ridgeline for quenched galaxies. Furthermore, the red spirals show similarly large central D n (4000), high [Mg/Fe], and dark matter halo mass to ellipticals. These results suggest that the bulges of red spirals formed within a short timescale before redshift ∼1–2 and were quenched via a fast mode, similar to ellipticals. Careful examinations of the optical morphologies reveal that ∼70% of red spirals show strong bars, rings/shells, and even merging features, which suggests that interactions or mergers might have played an important role in the formation of red spirals. In contrast, most of the massive blue spirals have completely different spectral and structural properties from red spirals. However, the blue spirals with high Σ1 ( ) show similar structural and morphological properties, as well as similar halo mass and H i mass to red spirals. We discuss rejuvenation from red to blue as a possible explanation for these high-Σ1 blue spirals.

The H i morphology and stellar properties of strongly barred galaxies: support for bar quenching in massive spirals

ABSTRACT Galactic bars are able to affect the evolution of galaxies by redistributing their gas, possibly contributing to the cessation of star formation. Several recent works point to ‘bar quenching’ playing an important role in massive disc galaxies. We construct a sample of six gas-rich and strongly barred disc galaxies with resolved H i observations. This sample of galaxies, which we call H i-rich barred galaxies, was identified with the help of Galaxy Zoo to find galaxies hosting a strong bar, and the Arecibo Legacy Fast Arecibo L-band Feed Array blind H i survey to identify galaxies with a high H i content. The combination of strong bar and high gas fraction is rare, so this set of six galaxies is the largest sample of its type with resolved H i observations. We measure the gas fractions, H i morphology and kinematics, and use archival optical data from the Sloan Digital Sky Survey to reveal star formation histories and bar properties. The galaxies with the lowest gas fractions (still very high for their mass) show clear H i holes, dynamically advanced bars, and low star formation rates, while those with the highest gas fractions show little impact from their bar on the H i morphology, and are still actively star-forming. These galaxies support a picture in which the movement of gas by bars can lead to star formation quenching. How these unusual galaxies came to be is an open question.

From massive spirals to dwarf irregulars: a new set of tight scaling relations for cold gas and stars driven by disc gravitational instability

ABSTRACT We present a new set of galaxy scaling relations for the relative mass content of atomic gas, molecular gas, and stars. Such relations are driven by disc gravitational instability, and originate from the low galaxy-to-galaxy variance of Toomre’s Q stability parameter. We test such relations using more than 100 galaxies, from massive spirals to dwarf irregulars, thus spanning several orders of magnitude in stellar mass ($M_{\star }\approx 10^{6\rm {-}11}\, \mbox{M}_{\odot }$) and atomic gas mass ($M_{\rm{H\, \small {I}}}\approx 10^{7\rm {-}10.5}\, \mbox{M}_{\odot }$). Such tests demonstrate (i) that our scaling relations are physically motivated and tightly constrained, (ii) that the mass-averaged gravitational instability properties of galaxy discs are remarkably uniform across the sequence Sa–dIrr, and (iii) that specific angular momentum plays an important role in such a scenario. Besides providing new insights into a very important topic in galaxy evolution, this work provides a simple formula (equation 5) that one can use for generating other galaxy relations driven by disc instability. We explain how to do that, mention a few possible applications, and stress the importance of testing our approach further.

A Break in Spiral Galaxy Scaling Relations at the Upper Limit of Galaxy Mass

Abstract Super spirals are the most massive star-forming disk galaxies in the universe. We measured rotation curves for 23 massive spirals with the Southern African Large Telescope (SALT) and found a wide range of fast rotation speeds (240–570 km s−1), indicating enclosed dynamical masses of (0.6−4) × 1012 M ⊙. Super spirals with mass in stars log M stars / M ⊙ > 11.5 break from the baryonic Tully–Fisher relation (BTFR) established for lower-mass galaxies. The BTFR power-law index breaks from 3.75 ± 0.11 to 0.25 ± 0.41 above a rotation speed of ∼340 km s−1. Super spirals also have very high specific angular momenta that break from the Fall relation. These results indicate that super spirals are undermassive for their dark matter halos, limited to a mass in stars of log M stars / M ⊙ < 11.8 . Most giant elliptical galaxies also obey this fundamental limit, which corresponds to a critical dark halo mass of log M halo / M ⊙ ≃ 12.7 . Once a halo reaches this mass, its gas can no longer cool and collapse in a dynamical time. Super spirals survive today in halos as massive as log M halo / M ⊙ ≃ 13.6 , continuing to form stars from the cold baryons they captured before their halos reached critical mass. The observed high-mass break in the BTFR is inconsistent with the Modified Newtonian Dynamics theory.

Peak star formation efficiency and no missing baryons in massive spirals

It is commonly believed that galaxies use, throughout Hubble time, a very small fraction of the baryons associated with their dark matter halos to form stars. This so-called low star formation efficiency f⋆ ≡ M⋆/fbMhalo, where fb ≡ Ωb/Ωc is the cosmological baryon fraction, is expected to reach its peak at nearly L* (at efficiency ≈20%) and decline steeply at lower and higher masses. We have tested this using a sample of nearby star-forming galaxies, from dwarfs (M⋆ ≃ 107 M⊙) to high-mass spirals (M⋆ ≃ 1011 M⊙) with HI rotation curves and 3.6 μm photometry. We fit the observed rotation curves with a Bayesian approach by varying three parameters, stellar mass-to-light ratio Υ⋆, halo concentration c, and mass Mhalo. We found two surprising results: (1) the star formation efficiency is a monotonically increasing function of M⋆ with no sign of a decline at high masses, and (2) the most massive spirals (M⋆ ≃ 1−3 × 1011 M⊙) have f⋆ ≈ 0.3−1, i.e. they have turned nearly all the baryons associated with their halos into stars. These results imply that the most efficient galaxies at forming stars are massive spirals (not L* galaxies); they reach nearly 100% efficiency, and thus once both their cold and hot gas is considered in the baryon budget, they have virtually no missing baryons. Moreover, there is no evidence of mass quenching of the star formation occurring in galaxies up to halo masses of a few × 1012 M⊙.

Magnetic fields in massive spirals: The role of feedback and initial conditions

Context. Magnetic fields play a very important role in the evolution of galaxies through their direct impact on star formation and stellar feedback-induced turbulence. However, their co-evolution with these processes has still not been thoroughly investigated, and the possible effect of the initial conditions is largely unknown. Aims. This Letter presents the first results from a series of high-resolution numerical models, aimed at deciphering the effect of the initial conditions and of stellar feedback on the evolution of the galactic magnetic field in isolated Milky Way-like galaxies. Methods. The models start with an ordered magnetic field of varying strength, either poloidal or toroidal, and are evolved with and without supernova feedback. They include a dark matter halo, a stellar and a gaseous disk, as well as the appropriate cooling and heating processes for the interstellar medium. Results. Independently of the initial conditions, the galaxies develop a turbulent velocity field and a random magnetic field component in under 15 Myr. Supernova feedback is extremely efficient in building a random magnetic field component up to large galactic heights. However, a random magnetic field emerges even in runs without feedback, which points to an inherent instability of the ordered component. Conclusions. Supernova feedback greatly affects the velocity field of the galaxy up to large galactic heights, and helps restructure the magnetic field up to 10 kpc above the disk, independently of the initial magnetic field morphology. On the other hand, the initial morphology of the magnetic field can accelerate the development of a random component at large heights. These effects have important implications for the study of the magnetic field evolution in galaxy simulations.